A conventional type of speed reduction means is structured as a certain train of gear means. As shown in FIG. 1 a gear means 2 linked to an output shaft of a motor is engaged with a smaller gear means 3 having a smaller number of teeth than that of the gear means 2. It is known that a reduced rotary output can be attained in proportion to the ratio of the number of teeth of the gear means 2 to that of the smaller gear means 3. Further, by mounting a smaller gear means 5 within an annular gear means 4 as shown in FIG. 2, it is known that the ratio of the number of teeth of the gear means 4 to that of the gear means 5 is a reduction gear ratio. Yet, since an input shaft and an output shaft in conventional gear-type speed reduction means are not placed on a coaxial line, it is inconvenient from various points of view to mount such reduction means in a normal power device. To avoid such inconveniences, it has been devised, as shown in FIG. 3, how to position the input shaft and the output shaft on a common coaxial line by means of planet gears 6, 6' engageable with a gear means 2' for input and a gear means 3' for output. Yet the reduction of speed by a multiple stage connection becomes complicated in structure, and the apparatus is oversized, expensive in cost and disadvantageous in durability.
Further, since the point of engagement of the input gear with the output gear, that is, the point of power transmission, is always the same, a very large load is applied to the teeth of gears. Accordingly, for maintaining a high load resistance and a constant rotary speed, a considerable strength and accuracy are necessary. Under these circumstances the manufacturing cost of such prior art reduction gear means becomes expensive.
The object of the present invention is to overcome the aforementioned disadvantages of the conventional art.